The present invention relates generally to polyolefin foams and, more particularly, to extruded and oriented foam sheets comprising polyethylene.
Polyethylene foam sheets and methods for manufacturing polyethylene foam sheets are well known in the art. See, e.g., U.S. Pat. Nos. 5,348,984 (Lee), 5,462,974 (Lee), and 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE). While LDPE possesses a number of beneficial physical and chemical properties when used to produce a foamed sheet, a disadvantage of LDPE is that extruded foam sheets made therefrom have a flexural modulus that is lower than would otherwise be desired for certain applications.
One method that has been proposed for improving the tensile strength, cushioning performance, and flexibility of extruded foams formed from a food product starting material is disclosed in JP 10-70973, which proposes that such foams be stretched immediately after extrusion at a take-off rate that is from 1.5 to 3 times higher than the extrusion rate.
The inventors hereof have found, however, that in the production of polyethylene foam sheets, a take-off rate/extrusion rate ratio (hereinafter referred to as a "draw ratio") ranging from 1.5 to 3 as taught in JP 10-70973 detrimentally reduces the dimensional uniformity of the resultant foam sheet. Such non-uniformity was most prominently manifested in the thickness dimension of the foam sheet, with a high degree of variation in the thickness of the foam occurring when the draw ratio was increased above 1.5. Such variation presents a poor aesthetic appearance and surface feel and, more significantly, indicates that the mechanical properties of the foam (strength, flexibility, etc.) are non-uniformly distributed throughout the foam. Any polyethylene foam sheets made in accordance with JP 10-70973 would therefore be commercially unacceptable.
Accordingly, a need exists in the art for a process of making polyethylene foam sheet having improved flexural modulus but with a low degree of variation in the thickness of such foam.